Field of the Invention
The present invention relates to a tag anti-collision method. More particularly, the present invention relates to a method of determining a frame size and a Medium Access Control (MAC) protocol capable of optimizing the use of a wireless resource in a multi-packet reception environment of a Radio Frequency IDentification (RFID) system.
Related Art
An RFID system is a contactless identification technology which identifies unique information of a tag attached to an object using a radio frequency. The RFID system includes a reader and a plurality of tags with identification information. The RFID tag has a very small size as compared with a smart card. The RFID tag is easily attached and inserted into various goods. A passive tag does not need power to be semi-permanently used. For this reason, the RFID system has been extensively applied to various industrial fields such as manufacturing, logistics management, military, retail, and transport management.
In the RFID system, in order to identify the tags, the reader sends an identification command to the tags. If the tags receive the identification command, the tags transmits a random 16-bit number (RN 16) generated thereby and unique identifier of the tags so that an identification procedure of the RFID system is performed. During the identification procedure, when a plurality of tags attempts the transmission simultaneously, collision is generated between transmission signals so that the reader has a difficulty in interpreting the received signals into data. Due to characteristics of the RFID system for identifying a plurality of objects within a short time, the above tag collision is a factor to increase an identification time in the RFID system. Accordingly, there is a need for a tag anti-collision protocol in order to solve a collision problem between tags.
A Framed Slotted ALOHA (FSA) based protocol is used as a representative anti-collision protocol and is a method that divides a time into frames including L slots having a fixed length where a tag transmits an RN16 and unique identifier of the tag in an arbitrary slot selected from the frame. The efficiency of the FSA based protocol is maximized when the number of tags to be identified is equal to a frame length L. However, it is difficult to change the frame length during a protocol operation. A Dynamic Framed slotted ALOHA (DFSA) has been developed to automatically adjust a frame length by estimating the number of tags to be identified. Further, an Adaptive Slot Adaptive Frame (ASAF) has been developed to reduce a time required for identifying the tags by reducing a length of an idle slot shorter than a length of a collision or success slot but slot lengths of the FSA and the DFSA are fixed.
In the anti-collision protocol according to the related art, it is assumed that a reader cannot identify data of tags due to collision between the tags when at least two tags simultaneously reply in the same slot. However, when a successive interference cancellation (SIC) is used, a plurality of original signals may be separated and identified from a composite signal received by the reader. This refers to a multi packet reception. There is a need to use at least two multi antennas for the purpose of the multi packet reception. In recent years, as a communication system using a multi antenna has been spotlighted, researches and studies have been performed toward a system performance when the multi antenna is applied to an RFID system. When the multi packet reception is used, unique identification information of an individual tag in a slot regarded as collision in the related art may be identified so that an identification rate of the tag is increased and a throughput of the RFID system may be improved. However, the existing anti-collision protocol does not take into consideration the above multi packet reception technology. EPC global Class 1 Generation 2 standard which is extensively used in a current RFID system is a single antenna based protocol. In addition, since it is regarded that only one tag may be successfully identified in one slot, it is regarded that the estimated number of tags successfully identified is less than a real number of successfully identified tags so that an optimal frame size cannot be determined.
In the anti-collision protocol, the reader replies an acknowledge (ACK) to the tags which are successfully identified in the slot. Since the existing anti-collision protocol regards that a tag is successfully identified only when only one tag transmits the RN16 and unique identifier in one slot, the ACK response of the reader is achieved for only a single tag. When a plurality of tags is successfully identified in one slot through the multi packet reception, the reader should transmit the ACK response to a plurality of tags. However, the existing anti-collision protocol recognizes the above case as collision so that the ACK response is not transmitted. Even if the ACK response is transmitted, since the ACK response may be transferred to only a single tag, thus other tags which are successfully identified in the same slot considers that the above tags are not identified. The RN16 and the unique identifier may be transmitted in a next frame when the ACK response is received.